This invention relates generally to the preparation of negative electron beam resists for photomask fabrication and/or semiconductor device fabrication and more particularly to halogenated polystyrenes useful in the preparation of said negative resists.
Briefly, a resist is an adhering layer of a material with patterned openings on a substrate which is used as a mask for etching, either partially or completely through, whereby the substrate is being exposed by the openings in the resist. Generally, resists are categorized as positive or negative.
The formation of positive resist masks from layers of radiation degradable polymers is described, for example, by Haller, Hatzakis and Srinivasan in U.S. Pat. No. 3,535,137. In this process, a radiation degradable polymer layer is coated on a substrate and is subjected to patternwise exposure to high energy radiation such as, for example, x-rays, deep UV, nuclear radiation or electrons. The irradiated regions of the polymer suffer a decrease in molecular weight and thereby become more rapidly soluble. A developer is then used to preferentially remove the irradiated portions of the layer. The substrate is then subjected to an additive or substractive process such as metallization or etching with the remaining portions of the resist layer acting to protect the substrate from the processing.
Alternatively, a negative resist mask is formed by coating a cross-linkable polymer on a substrate. The polymer is generally applied as a solution in a suitable solvent to the substrate and allowed to dry as a thin film. High energy radiation, such as an electron beam, short wavelength UV light, or X-rays, is permitted to irradiate the surface of the polymer in the desired pattern to form a negative resist by imparting sufficient energy to cross-link the polymer. Cross-linking causes the irradiated portion to be insoluble in certain solvents. After irradiation is completed, the resist is subjected to a solvent and the nonirradiated portion of the resist is removed, thereby leaving openings in the resist that correspond to the desired pattern. The present invention relates to negative resists.
Prior art negative resist materials have several shortcomings, among which is the difficulty in obtaining sharp images of high resolution. Moreover, deep UV lithography using the wavelength range from 2000 to 3500 .ANG. is receiving considerable attention as a possible economic mass production method of manufacturing ultra fine patterns (about 1 .mu.m) for making VLSI. As these deep UV tools become available, much emphasis is being placed on finding compatible negative resist-materials with high resolution, high contrast and plasma resistance.
Recently, polystyrenes have been receiving renewed attention as electron and X-ray resists, primarily since it has been shown by others that very high contrast can be obtained with high molecular weight polystyrene exposed to electron beams at doses in the range of 10.sup.-4 coul/cm.sup.2 (100 .mu.coul/cm.sup.2). It has also been shown that the sensitivity of polystyrenes can be considerably improved by physical doping with cross-linking agents and sensitizers or by incorporating such elements as chlorine, bromine or iodine in the polymer chain. In this regard, see, for example, J. C. Jagt and A. P. G. Sevriens, Photopolymers Principles--Processes and Materials, SPE, Ellenville, NY, p. 152 (1979); T. Tsunoda, et al., Phot. Sci. & Engr., 17(4), 390 (1973); N. A. Weir and T. H. Milkie, J. Poly. Sci. (Chem), 17, 3735 (1979); W. Burlant, et al., J. Poly Sci., 58, 491 (1962); E. D. Reit, Photopolymers Principles--Processes and Materials, SPE, Ellenville, NY, p. 91 (1979); and H. Shiraishi, et al., Photopolymers Principles--Processes and Materials, SPE, Ellenville, NY, p. 56 (1979). However, this literature illustrates that even though the sensitivity of the polystyrenes was increased in the 10.sup.-6 coul/cm.sup.2 range, residues and swelling of the exposed pattern are always experienced when trying to develop a high resolution pattern in a relatively thick resist layer (.about.1 .mu.m).